The 3rd Generation Partnership Project, 3GPP, is responsible for the standardization of the Universal Mobile Telecommunication System, UMTS, and Long Term Evolution, LTE. LTE is also sometimes referred to as Evolved Universal Terrestrial Access Network, E-UTRAN. LTE is a technology for realizing high-speed packet-based communication that can reach high data rates both in the downlink and in the uplink, and is a next generation mobile communication system relative to UMTS. LTE brings significant improvements in capacity and performance over previous radio access technologies.
The Universal Terrestrial Radio Access Network, UTRAN, is the radio access network of a UMTS and Evolved UTRAN, E-UTRAN, is the radio access network of an LTE system. In an UTRAN and an E-UTRAN, a User Equipment, UE, is wirelessly connected to a Radio Base Station, RBS, commonly referred to as a NodeB, NB, in UMTS, and as an evolved NodeB, eNB or eNodeB, in LTE. An RBS is a general term for a radio network node capable of transmitting radio signals to a UE and receiving signals transmitted by a UE. The area served by one or sometimes several RBSs is referred to as a cell.
The ever increasing end-user demands are a significant challenge to the operators. Separating users spatially by means of precoding and/or beamforming is one way to improve the performance of a wireless system.
Although only Multiple Input Multiple Output, MIMO, systems up to 8 or 16 antennas are supported by existing standards and so called MIMO transmission modes, the use of very large antenna arrays (also called massive or “Full Dimension” MIMO systems) in commercial cellular systems has been proposed only recently. The 3rd Generation Partnership Project, 3GPP, is currently working on the implications of supporting up to 64 transceiver units, TXRU, to serve many users simultaneously (so called multiuser MIMO) and/or create narrow (pencil) beams to scheduled users. See 3GPP TR: 36.897: Study on Elevation Beamforming/Full-Dimension, FD, MIMO for LTE: version 0.2.1.
The expectation of massive MIMO systems is a boost of the spectral efficiency, capacity as well as providing a uniform user experience as opposed to large variations in the received user bit rate or quality of service between cell center and cell edge areas. The underlying theory of massive MIMO systems is that under the assumption of perfect channel estimation, the vector channel of a served user grows orthogonal to other users and thereby interference can be virtually eliminated.
Downlink transmission methods rely on knowledge of the channel at the transmitting base station, BS, or, more precisely, the availability of estimates of the channels between the BS antennas and the wireless devices to which this BS is transmitting information. This channel state information is then used to “precode” the information intended for each of the wireless devices prior to transmission, in such a way, that each of the wireless devices is able to decode the signals of its own interest.
The necessary channel state information is obtained by transmitting pilots, i.e., known signature waveforms, over the wireless medium and estimating these channels based on the received waveforms. Then these estimates are used for generating the MIMO precoder (i.e., the transmission method) and for transmitting data to the wireless devices.
However, pilot sequences represent a limited resource, because the length, number of symbols, of pilot sequences is limited by the coherence interval and bandwidth of the wireless channel. In turn, the number of orthogonal pilot sequences and thereby the number of separable users is limited by the length of the available pilot sequences. Consequently, when the number of antennas grows large, the number of spatially separable users is not limited by the number of antennas but the number of available orthogonal pilot sequences. Therefore, in multicell systems, the pilot sequences must be reused which unavoidably leads to interference between identical pilot sequences. This interference in multicell massive MIMO systems is known as the pilot contamination problem.
FIG. 1 illustrates that if a pilot is reused in adjacent cells; there may be interference, because the pilots transmitted in different cells may be overlapping. Hence, the base station 110a cannot differentiate the pilots transmitted by the two UEs 10a and 10b. 
In massive MIMO systems, the so called pilot contamination, PC, or multicell pilot signal interference problem is known to degrade the quality of channel state information, CSI, at the BS, which in turn degrades the performance in terms of actually achieved spectral efficiency, beam forming gains and cell edge user throughput.
A well-known prior art technique is to avoid pilot sequence reuse-1 in neighbor cells and thereby maintain user separation in the code domain. Pilot sequence reuse-1 (or full reuse) implies that all pilot sequences are reused in every cell, which should be compared to pilot sequence reuse-2, where the effective pilot cell-reuse equals 2, i.e. each pilot is reused in every 2 cells or pilot sequence reuse-3, where the effective pilot cell-reuse equals 3, i.e. each pilot is reused in every 3 cells. The basic idea is similar to higher frequency reuse schemes known in GSM systems.
Another prior art technique proposes a solution to the pilot contamination problem based on multicell cooperation to achieve spatial separation between users who use the same pilot sequence. According to the prior art, the cooperating cells exchange long term CSI and perform a coordinated pilot assignment to users. A long term CSI is essentially the average of the user channels over some time window. The long term CSI is exchanged in the form of the so called covariance matrices of user vector channels. The coordinated pilot assignment to users is performed such that spatially well separated users are assigned identical pilot sequences in neighboring cells. By spatial separation, the impact of intercell interference and thereby pilot contamination is mitigated.
The problem with the state of the art multicell coordination based scheme is three-fold:                Cooperating network nodes such as cellular base stations or wireless access points need to measure, estimate and subsequently exchange channel covariance matrices. The exchange of such covariance matrices is problematic, because the size of such matrices grows quadratically with the number of antennas, whereas the actual number of the matrices grows linearly with the number of users and the number of interfering base stations;        The estimation or measurement of the covariance matrices is problematic due to the issue of long term changes in the user channels, due to changes in the long term geometry of the system due to mobility, environmental changes in the propagation conditions, etc.        The processing of the covariance matrices to determine the spatially well separated user sets imposes a computational burden on the network nodes participating in the cooperation due to the frequent updates and to the computational burden of determining pilot assignment based to the received covariance matrix information.        
For further reading see “A Coordinated Approach to Channel Estimation in Large-scale Multiple-antenna Systems” H Yin, D Gesbert, M Filippou, Y Liu IEEE Journal on Selected Areas in Communications 31 (2), 264-273.
Hence, although in theory multicell cooperation can help mitigate PC effects, its fundamental input, e.g., user channel covariance matrix acquisition, exchange and processing, renders it problematic in practical systems for example due to the above problems.
Hence, there is a need for improved methods for mitigating pilot contamination.